The present invention relates to a flow control system for use in a power steering apparatus, and more particularly it relates to a system for use in a power steering apparatus including a fluid cylinder for amplifying the manual steering torque and a control valve associated therewith, said system serving to control the flow rate of pressure fluid being fed from a hydraulic pump to the fluid cylinder via the control valve, in response to the vehicle speed.
It is preferable for a power steering apparatus to be capable of decreasing its steering torque amplifying function when the vehicle is running at high speed. As an example typical of such function, an arrangement is known which uses a pump, usually called a dropping pump, which provides a flow rate characteristic indicated by a curve A in FIG. 1. (In addition, a curve B in FIG. 1 indicates the characteristic of a proportional discharge pump wherein the discharge rate Q increases in proportion as the pump rpm N increases, and a curve C indicates the characteristic of a constant discharge pump wherein the discharge rate increases linearly until a certain value of the pump rmp is reached, whereupon the discharge rate Q becomes constant even if the rpm N increases.) The dropping pump has a function such that the discharge rate increases linearly until a certain value of the rpm is reached, whereupon the discharge rate becomes constant until another certain value of the rpm is reached, and then the discharge rate Q decreases with increasing rpm N.
However, since such pump is driven directly by the rotation of the engine, there is a drawback that the discharge rate Q varies with the engine rpm and does not necessarily agree with the vehicle speed.
With the object of varying the flow rate with the vehicle speed, there have been proposed various flow control systems, as found, for example, in Japanese Patent Publication No. 39529/1977, and Japanese Patent Disclosure Nos. 30039/1972 and 41326/1977.
These proposals may be classified into two types, one in which when the vehicle speed reaches a certain value, the flow control valve is on-off operated, and the other in which the flow rate is continuously varied in accordance with the vehicle speed. In the former type, the stepwise changes in the flow rate by said one-off operation are so large that such sharp changes can give the driver an uneasy feeling, and in the latter type, the construction of the control valve is extremely complicated in order to obtain the predetermined flow rate characteristic and there are many problems in practice. Further, in eithre case, little or no consideration has been given to safety measures against rare faults or to measures against pulsations of pressure fluid.
As for methods of electrically effecting the flow rate control of the power steering apparatus, mention may be made of Japanese Patent Publication No. 3142/1976 wherein a vehicle speed signal (frequency) is converted into a change in current and changes in the attraction of an exciting coil produced by increases and decreases in current are utilized, and Japanese Patent Publication No. 27887/1977 and Japanese Utility Model Publication No. 6828/1976 wherein the vehicle speed is converted into a voltage by a tachometer generator and increases and decreases in voltage are used to actuate a plurality of solenoids to control the flow rate. In these methods, however, the change of the steering characteristics which should attend the change of the type and specification of the vehicle is difficult to make, detracting from versatility, and despite the fact the amount of control fluid is stepwise changed, the number of control stages is limited by the weight and the space available, so that changes in the vehicle speed during run cause sharp changes in the steering torque, thus giving the driver an uneasy feeling. Moreover, in each of these methods, little consideration has been given to a measure for a fail-safe feature against rare faults or to a measure for prevention of mulfunction in steering, and even if the electric circuitry is relatively simple, the mechanism associated with the valve section is complicated and the weight is increased, adding to the cost.